here is kayak/recumbent bicycle rig.
pretty close to my lightweight moped motor version.
http://www.youtube.com/watch?v=zoSLqm1aVvA&feature=related
too bad his video doesn't include it in action. My question is does it go from floating to land balancing without getting out and walking it up the ramp.
However, even if you need to do that the idea still has merit, and the dismount would get you an opportunity to take off life vest and don a bike helmet, and swap your plastic flip flops for shoes with socks.
pretty easy to envision it with a 50cc moped motor, belt drive to wheels geared down from 35mph+ to under 15 mph(for more ramp climbing ability).
I'd like retracting wheels for speed in water(I imagine a 50cc motor would be plenty for any kayak), but my old mountain bike(18sp) has been ridden in sandy surf repeatedly, with sand bearing salt water all up in the gears 3ft deep and it seems none the worse for wear, so soaking bike running gear not a total disaster.
These cat-kayaks might be good starting point for recumbent or even up-ride bike-moped/kayak hybrid, and might be easy to have retractable/liftable wheels front and back and not butcher the boat in the process....and be able to just unclamp a front and back rig and be back to 'boat only'.
http://www.wavewalk.com/